For the human rights groups that work hard to bring international attention to the abuses at North Korea’s network of vast prison camps, Monday’s call for an international inquiry into the camps by the United Nations’ human rights chief marked a breakthrough moment.

More-visible human rights violations in countries such as Syria tend to hog media attention, despite the significantly larger scale of the problem in North Korea. Political interest in North Korea is usually concentrated on the threat it represents to neighboring countries rather than to its own people.

“Their personal stories were extremely harrowing,” Ms. Pillay said. “They described a system that represents the very antithesis of international human rights norms.”

North Korea’s camps are estimated to hold around 200,000 people who are subjected to brutal living conditions, forced hard labor, torture and malnutrition. Pyongyang denies the existence of any camps despite first-hand testimony and detailed satellite imagery.

“For Citizens’ Alliance and me personally this is very important because it’s been 10 years since we started to go to Geneva to draw attention to human rights abuses in North Korea,” said Joanna Hosaniak, head of international campaigns at the group.

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Shin Dong-hyuk, who spent 23 years in a North Korean prison camp.

“At that time, the environment was very hostile and not many diplomats wanted to talk to us,” she said.

To move the process forward now, a resolution calling for a commission of inquiry needs to be submitted to the United Nations Human Rights Council meeting in March.

Japan or the U.K. are viewed as potential sponsors, although both countries are likely to be wary of the backlash from North Korea of taking such a step. The U.K. maintains an embassy in Pyongyang and runs cultural programs in the North, while Japan has been in negotiations over the unresolved issue of Japanese nationals abducted by the North.

Ms. Pillay’s call for an inquiry is the latest ramp-up of international pressure on North Korea over human rights. Last year, the general assembly of the UN passed a resolution condemning North Korea’s human rights abuses by consensus and without any opposition from China, the North’s main supporter. The U.S. also extended its North Korea human rights act through 2017 last year, while Japan has similar legislation and the EU regularly raises human rights problems in North Korea.

For outsiders, one puzzling question is why South Korea doesn’t apply more pressure on North Korea over human rights, including those of South Korean nationals held in the North. Legislation addressing the problem occasionally gets submitted to parliament but is routinely blocked by left-of-center parties over concerns it will upset the North. There was no substantial debate over human rights in North Korea during the race for South Korea’s presidency last year.

Ms. Pillay also called for North Korea to clarify the fate of many Japanese and South Korean nationals abducted or forcibly detained by the North.

South Korea’s Unification Ministry, which handles relations with North Korea, said it appreciated the UN’s involvement in the issue.

“We are well aware of the human rights situation in North Korea and we welcome the international community’s efforts on this issue,” a spokesman said.